1. Field of the Invention
The present invention relates to a liquid lens for adjusting a focal length by electro-wetting, and more particularly, to a liquid lens with an improved sealing structure, in which a glass cover and a body are bonded together by two types of adhesives with different compositions, thereby significantly enhancing reliability of the final lens product.
2. Description of the Related Art
In general, a liquid lens is known to adjust a focal length by electro-wetting.
As shown in FIG. 1, the conventional liquid lens 200 includes an electrolyte 220 disposed on an insulator 210 and an electrode 230 disposed underneath the insulator 210. When current is supplied to the electrode 230 and the electrolyte 220, an interface angle θ between the electrolyte 220 and the insulator 210 is altered.
That is, a drop of electrolyte, if present on a surface of the insulator 210, forms interfaces between the insulator 210 and the electrolyte 220, the electrolyte 220 and gas 225, and the insulator 210 and the gas 225. Out of these, the electrolyte 220 and the insulator 210 have an interface angle θ therebetween according to following Young's equation.γSL−γSG=γLG COS θ  Young's equation
where S denotes the insulator, L.denotes the electrolyte, G denotes the gas and γ denotes surface tension coefficient.
Here, the electrolyte 220 is made of a conductive fluid and the insulator 210 in contact therewith is made of an insulating film. Then the surface tension coefficient is altered when a voltage is applied to an electrode 212 formed below the insulator 210 and to an electrode 222 formed in the electrolyte 220. The surface tension coefficient is determined according to following Lippmann's equation.γ=γ0−(½)cV2   Lippmann's equation
As described above, the surface tension coefficient is changed according to the voltage V applied and a dielectric constant c of the insulating film. Then, the interface angle θ is altered by the change in the surface tension coefficient γSL between the insulator 210 and the electrolyte 220.
FIGS. 2 and 3 illustrate a conventional liquid lens 300 adopting such a basic principle.
In the liquid lens 300, a body 305 is made of a conductive material and has an inner space 305a in a central portion thereof. A transparent substrate 310a is disposed underneath the body 305. The inner space 305a is filled with a non-conductive fluid 330 of oil and a conductive fluid 350 of electrolyte, which both are substantially identical in density. A transparent cover 310b is bonded onto the body 305. Also, a light transmissible electrode 33 is disposed underneath the cover 310b. 
Furthermore, insulating layers 360 are formed on both inner walls of the inner space 305a to extend along an upper surface of the body 305. Another electrode 352 is formed on the body 305 to connect to a power voltage and a voltage is applied to the conductive fluid 350 through the electrode 332.
As a result, in the conventional liquid lens 300, a voltage is adjustably applied to the conductive fluid 350 through the electrodes 332 and 352. This alters an interface angle θ between insulating layers 360 and the conductive fluid 350, as shown in FIG. 3, thereby changing a shape of the interface between the conductive fluid 350 of electrolyte and the non-conductive fluid 330 of oil. This accordingly alters a focal length of light propagating through the interface.
Vigorous attempts have been made to commercialize the conventional liquid lens 300, whose final product, however, was hardly reliable.
In the conventional liquid lens 300, the body 305 made of a conductive material is filled with the non-conductive fluid 330 of oil and the conductive fluid 350 of electrolyte. Then the cover 310b is capped on the body 305 by using a bonding device (not illustrated). The bonding device applies an ultraviolet ray curing adhesive 370 along an outer periphery of the cover 310 so that the adhesive 370 is penetrated by a capillary phenomenon and cured to bond the cover 310b to the body 305.
While the ultraviolet ray curing adhesive 370 is penetrated between the cover 310b and the body by the capillary phenomenon and cured, the adhesive penetrating in an excessive amount is trapped inside adhesive blocking grooves 372 formed in the upper surface of the body and adjacent to the inner space 305a. The grooves 372 block the adhesive 370 from reaching the non-conductive fluid 330 and the conductive fluid 350 inside the inner space 305a, which are thus free from any effects of the adhesive 370.
The conventional liquid lens 300 lacks reliability. That is, a bonding portion between the cover 310b and the body 305 is limited to a small area, i.e., the outer periphery of the upper surface of the body 305. This structural vulnerability degrades durability. Moreover, heat generated from curing of the ultraviolet ray curing adhesive 370 expands the non-conductive and conductive fluids 330 and 350, weakening the bonding portion between the cover 310b and the body 305.